I bought an adapter but the top volume level is pitifully low, so I'm back to burning CDs to play in my car.

This is odd, because the voltage levels should be somewhat normalized across the USB-C adapter and your old headphone jack. It may be an issue with your adapter having a shitty DAC. Basically, the adapter has to take the digital audio signal, and convert it to analog. Cheaper adapters will use cheap digital-analog converters (DACs) which will either output lower levels, or will tend to change the signal as volume increases.

It’s also possible that it is purely an analog converter, in which case your phone is actually using its internal DAC. There are benefits and drawbacks to this, but it’s possible that your phone is software-limiting its internal DAC’s power output to avoid burning out from a bad connection.